This invention relates generally to engine powered vehicles having power shifting, hydraulic fluid transmissions and more particularly to systems for applying vehicle brakes. Means is provided to sense a pressure signal of hydraulic fluid that is proportional to ground speed movement of a vehicle. Control means transforms the signal generated by fluid pressure into means for applying vehicle brakes in a manner proportional to vehicle ground speed, either in a forward or reverse direction. Dual pressure regulating means are provided wherein one means is set at a higher pressure than the other means for eliminating a need for two sources of pressure and/or a pressure accumulator.
For convenience of illustration, the invention will be described with reference to its use in a steering and brake control system for construction machinery, such as a crawler tractor, but it is believed to be applicable and have general utility in any steering-by-driving vehicle.
Control of braking and steering systems used in large steering-by-driving vehicles, such as crawler tractors, has been for many years a main concern of engineers and other technical persons skilled in the art. A great many examples of systems and apparatus to solve problems in this field of use have been disclosed in a number of prior art patents. Two attempts to solve these problems may be found in U.S. Pat. Nos. 3,765,454, issued on Oct. 16, 1973 in the name of David F. Carl and U.S. Pat. No. 3,837,449, issued on Sept. 24, 1974 in the name of Gary A. Drone, both patents assigned to the assignee of the present invention. The first named reference is concerned primarily with modulating the rise of pressure in fluid directed to a steering clutch. The second named reference is directed to structure that automatically applies a vehicle brake when transmission fluid pressure drops because of shifting to neutral or the engine stops.
U.S. Pat. No. 4,230,152, issued on Oct. 28, 1980 in the name of Carl, et al, a multi-function control valve discloses a hollow, ported valve spool movable in response to selective operation of a manually actuated valve spool to effect flow modulation from a first fluid source through the valve. One end of the hollow, ported valve spool engages a plug portion of a freely floating piston movable in response to fluid pressure from a second fluid source to close a normally open port in the hollow, ported valve spool.
While D. F. Carl, et al, U.S. Pat. No. 4,230,152 discloses a control valve system for braking vehicles that is highly satisfactory for steering-by-driving applications, it relies on pressure applied by a foot operated pedal to move a piston within a spool valve to shut off fluid pressure. In addition, there is required a dual pump structure in order to provide a first fluid flow through the valve and a second fluid source to communicate fluid to a small spool in order to shut off fluid pressure. Further, this structure is complicated in that there are too many interfaces that can cause mixing of steering and transmission fluids. In addition, this system fails to provide for means to accomplish proportional braking.